Note 4 - Employee Stock Benefit Plans (Details Textual) $ / shares in Units, $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	49,800
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance	422,426
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Ending Balance \| $ / shares	$ 10.19
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value \| $	$ 6.0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	4 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	422,426
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price \| $ / shares	$ 10.19
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value \| $	$ 6.0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	4 years
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	284,808
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	427,227
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance	808,093
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding \| $	$ 19.8
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms	1 year 182 days
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	88,418
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	40,640
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance	339,771
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding \| $	$ 8.3
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms	1 year 219 days
Equity Based Performance Stock Units Granted in 2018, 2017, and 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Equity Based Performance Stock Units Granted in 2018, 2017, and 2016 [Member] \| Vest on the Third Anniversary of Awards Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	100.00%
Equity-based Performance Stock Units Granted in 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award, Performance Criteria Period	2 years
Equity-based Performance Stock Units Granted in 2015 [Member] \| Vest on the Second and Third Anniversary of Awards Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	50.00%
Minimum [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	1 year
Minimum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	1 year
Minimum [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Shares Available for Issue	25.00%
Maximum [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Maximum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Maximum [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Shares Available for Issue	200.00%
Equity Incentive Plan 2005 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,203,313
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	559,646
Percentage of Fair Value to Determine Price of Common Stock	85.00%
Stock Issued During Period, Shares, Employee Stock Purchase Plans	41,694

Percentage of the Fair Value to Determine the price of common stock.

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In a performance-based equity compensation plan, the timing in which performance goals are set relative to the grant date.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Intrinsic value of equity-based compensation awards outstanding. Excludes stock and unit options.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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